Flip-flop lightning arrester with improved means for preventing parallel operation



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FLIP-FLOP LIGHTNING ARRES W IMPROVED MEANS FOR PREVENTING PARALLEL OPERATION Filed Sept. 16, 1968 2 Sheets-Shepl M z 4? f1 44196726 Chafing; Jam; ST/ re e;

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Nov. 3,1970: msK JR AL 3,538,387

v FLIP-FLOP L TNING ARRESTER W ROVEI), MEANS FOR PREVENTING PARALLEL OP \ATION v l Filed Sept. ,16, 1968 2. Sheets-Sheet '2 (hi/761$ 67065 6, 4y I Z41 fi fie/r fame United States Patent Office 3,538,387 Patented Nov. 3, 1970 US. Cl. 317-61 20 Claims ABSTRACT OF THE DISCLOSURE A flip-flop lightning arrester in which a magnetic arc blow-out coil of a current limiting gap assembly of one of a number of electrical parallel flip-flop legs is shunted by a valve resistor and a coil of the current limiting gap assembly of another leg is shunted by a gap. The coil that is shunted by a valve resistor may have a lower impedance than the coil that is shunted by a gap. The lower impedance may be produced by lower inductance or lower resistance or both. Alternatively, or additionally, the legs may also include unequal series valve resistance and in the additional case the lower series valve resistance is in the leg whose current limiting gap assembly coil is shunted by a valve resistor.

This invention relates to lightning arresters and more particularly to improvements in so called flip-flop lightning arresters.

A flip-flop lightning arrester comprises at least two electrically if not physically parallel branch circuits, columns or legs each comprising at least one current limiting spark gap or gap assembly. A current limiting gap is a gap which after spark over builds a counter voltage comparable to its spark over voltage in a time which although long in comparison to the duration of the front of an impulse voltage such as a lightning surge, is short in comparison to the duration of a switching surge on a high voltage power transmission system. Such lightning arresters are particularly useful in discharging long duration high energy switching surges on high voltage direct current power transmission systems. This is because in normal operation, the switching surge discharge current is cyclically transferred back and forth from one column to the other, hence, the name, flip-flop. This flip-flop operation prevents overheating of the current limiting gap so that the arrester can discharge more energy than if all the energy were continuously discharged through the gaps.

However, it is the unfortunate possibility that both legs of a flip-flop arrester may spark over simultaneously in response to an over-voltage. It is also true that if an arrester leg, in which the gap units are equipped with a voltage grading circuit, is conducting, that leg will not be capable of sparking itself over at the proper level, therefore if initial simultaneous leg sparkover occurs and if both legs continue to conduct after the gaps have developed significant arc voltage, the arrester will not function in its intended manner. The result will be that leg ent rates of voltage build-up by modifying the geometry of the arc chambers of their current limiting gaps so as to prevent this unwanted and self defeating parallel op eration. This is disclosed and broadly claimed in app ication Ser. No. 624,297, filed Mar. 20, 1967 as a continuation-in-part of an original application Ser. No. 553,413, filed May 27, 1966, in the names of Eugene C. Sakshaug, and James S. Kresge and assigned to the assignee of this application.

In accordance with this invention, simpler, less expen sive, more effective and reliable means are provided for giving the legs different rates of countervoltage build-up for preventing any such harmful parallel operation. Briefly and generally speaking such means comprises electric circuit element changes rather than essentially physical changes in arc chamber geometry.

An object of the invention is to provide a new and improved lightning arrester.

Another object of the invention is to provide improved means for preventing parallel operation in flip-flop lightning arresters.

A further object of this invention is to provide new and improved means for giving the parallel legs of a flip-flop lightning arrester significantly different rates of countervoltage build-up.

The invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1 is a schematic, diagrammatic representation of a preferred embodiment of the invention, and

FIGS. 2, 3, 4 and 5 are sets of curves or graphs for illustrating the operation of the invention.

Referring now to the drawings, and more particularly to FIG. 1, there is shown therein schematic and diagrammatic form a flip-flop lightning arrester 1 connected by way of example between a power system conductor 2 such as a high voltage direct current conductor and ground indicated at 3. The arrester is shown as comprising parallel legs A and B, each comprising a current limiting gap or gap assembly. Current limiting ga-p A comprises, by way of example, four magnetic blow-out horn gaps 4 4 4 4 serially connected with each other and with a magnetic blow-out coil 5. Likewise, current limiting gap B comprises four magnetic arc blowout gaps 4' 4' 4' 4' serially connected with each other and with a magnetic blow-out coil 5'. Coil 5 is shunted by a protective gap 6 of the type which also builds voltage due to magnetic blow-out action on its arc. Coil 5' of the gap B however, is shunted by a comparatively small valve resistor 7 which may typically be a small piece of valve resistance material about 3" in diameter and A" thick. Preferably, the impedance of coil 5 is made higher than the impedance of coil 5 by having its resistance and/or its inductance higher than that of coil 5.

Also included as a part of the current limiting gap assembly A is a voltage grading network comprising impedances 1, Z Z Z connected in shunt with the individual main series gaps 4 4 4 4 Likewise, the current limiting gap B comprises a similar voltage grading network comprising impedances Z Z Z' Z' connected respectively in shunt with the main series gaps 4' 4' 4' 4' Although not essential, each leg also includes a series valve resistor shown as resistor 8 for leg A and resistor 8' for leg B and also preferably the resistance of resistor 8 is higher than the resistance of resistor 8'.

The arrester is also shown as comprising a common leg consisting of a common valve resistor 9.

In the event of simultaneous leg sparkover the operation is illustrated in FIGS. 2, 3, 4 and 5, each of which consists of a left hand graph A pertaining to leg A and a right hand graph B pertaining to Leg B. All of them have the same horizontal time scale covering roughly 1,000 micro-seconds. After parallel sparkover, current starts to flow through the arrester and is distributed between the two legs as shown by FIGS. 2A and 2B. In leg B, current flow immediately produces significant voltage drop across the valve resistor 7 and the coil This is shown in FIG. 3B. This voltage causes current to build up rapidly in the lower inductance, lower resistance coil 5. Magnetic flux is established by coil 5 and increases proportionally to the coil current as shown in FIG. 43 causing the arcs of the gaps 4' to move into their arc chambers and develop voltage rather quickly as shown by FIG. 5B.

In leg A that has the gap 6 shunting the coil 5 events take place at a slower pace. During spark over of this leg, the gap 6 is sparked over. Initial voltage across this gap is very small as shown in FIG. 3A because, of course, the coil voltage is the same as the voltage of gap 6 the two being directly in parallel. Consequently, the resulting magnetic flux build-up of the coil 5 is relatively slow as shown in FIG. 4A. This is further enhanced by the relatively high resistance and inductance of coil 5. Therefore the arc in the coil gap 6 is elongated relatively slowly. Similarly, the arcs of the main gaps 4 are extended or elongated relatively slowly. Not until the coil gap 6 are becomes fully extended does its voltage become fully significant. Then the gap units get full flux and the gap unit develops meaningful arc voltage.

The gaps of leg B reach this condition sooner while the above process is only partially completed for the gaps of leg A. Because of this, the arc voltage of the leg B can force complete current transfer to leg A. Thus it will be seen that at time T in FIG. 2B, the current in leg B has been forced to zero by the rapid build-up of voltage in the current limiting gap B as shown in FIG. 5B. It will be noted that at the same time T in FIG. 5A, the voltage of the current limiting gap for leg A is relatively low and the leg current as shown in FIG. 2A is relatively high thus all of the arrester current is forced out of leg B and into leg A in the matter of a few hundred micro-seconds, and this permits leg B to clear or seal 01f against the circuit voltage. As will also be observed in FIG. 5A, this forced transfer does not produce excessive leg voltage because the voltage developing capability of the coil gap in leg A is as yet incomplete and the voltage of gap unit A is quite low at time T when the current is transferred and parallel operation ceases.

Successful clearing of the leg B is aided by the fact that in clearing the current limiting gap B does not extinguish the current in its coil 5. The current in the coil 5 is free to flow through the parallel valve resistor 7. Thus the magnetic flux does not decrease to zero at the same time as the gap current, but slightly later. This prevents the clearing process from slowing down as the leg current reaches low levels. This is illustrated in FIGS. 3B and 4B where the coil voltage reverses after time T and continues to drive current through the coil and the resistor 7 so that the magnetic flux does not fall to zero at T This is no the same as for the coil of gap A.

The different resistance values of the valve resistors 8 and 8' also cause different rates of arrester voltage development in the legs 8A and B. This unbalance causes more current to flow in leg B than in leg A when initial parallel operation occurs. The gaps with the smaller current will tend to develop voltage slower and the gaps with the larger current, faster. This is believed to be because at higher currents the arcs are denser and therefore magnectic flux produces a higher force on them. In addition the effect of the local magnetic field produced by current flow in the gap electrodes is to drive the arc toward the arc chamber. This effect increases with increasing current.

It should be understood that the valve resistors 8 and 8' do not necessarily have to have unequal resistance and Cit equal resistors can be used with the feature of having one of the coils shunted by a gap and the other by a valve resistance.

Alternatively, when unequal valve resistors are used in the legs, it is generally desirable but may not be necessary to use unequal current limiting gaps in the legs.

Even if the flip-flop lighting arrester has more than two legs it is still possible that simultaneous spark-over of all the legs may occur. Therefore, it should be understood that the present invention is not limited to a two-leg flipfiop arrester and that regardless of the number of legs that are employed, at least one of them should correspond to leg B, the others being like leg A.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention and therefore it is intended by the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a flip-flop lightning arrester having at least two parallel circuits each comprising one current limiting gap assembly with a respective series magnetic arc blow-out coil, one of said coils being shunted by a gap and the other being shunted by a valve resistor.

2. In a flip-flop lightning arrester having at least two parallel circuits each comprising at least one circuit limiting gap assembly with a respective series magnetic arc blow-out coil, one of said coils having significantly higher impedance than the other.

3. An arrester as in claim 2 in which the higher impedance coil has the higher inductance.

4. An arrester as in claim 2 in which the higher impedance coil has the higher resistance.

5. An arrester as in claim 2 in which the higher impedance coil has the higher inductance and resistance.

6. An arrester as in claim 2 in which the lower impedance coil is shunted by a valve resistor and the higher impedance coil is shunted by a gap.

7. An arrester as in claim 3 in which the lower inductance coil is shunted by a valve resistor and the higher inductance coil is shunted by a gap.

-8. An arrester as in claim 4 in which the lower resistance coil is shunted by a valve resistor and the higher resistance coil is shunted by a gap.

9. An arrester as in claim 5 in which the lower inductance and resistance coil is shunted by a valve resistor and the higher inductance and resistance is shunted by a gap.

10. In a flip-flop lighting arrester having at least two parallel circuits each comprising at least one current limiting gap assembly in series with a valve resistor, each of said gap assemblies comprising a series magnetic arc blow-out coil, one of said valve resistors having significantly higher resistance than the other.

11. An arrester as in claim 10 in which the coil in the higher series valve resistance circuit is shunted by a gap and the coil in the lower series valve resistance circuit is shunted by a valve resistor.

12. An arrester as in claim 10 in which the coil in the higher series valve resistance circuit has a higher impedance than the coil in the lower valve resistance circuit.

13. An arrester as in claim 11 in which the coil in the higher series valve resistance circuit has a higher impedance than the coil in the lower series valve resistance circuit.

14. An arrester as in claim 12 in which said higher impedance coil has a higher inductance than the other coil.

15. An arrester as in claim 13 in which said higher impedance coil has a higher inductance than the other coi 16. An arrester as in claim 12 in which said higher impedance coil has a higher resistance than the other coil.

17. An arrester as in claim 13 in which said higher impedance coil has a higher resistance than the other coil.

18. An arrester as in claim 12 in which said higher impedance coil has a higher inductance and resistance than the other coil.

19. -An arrester as in claim 13 in which said higher impedance coil has a higher inductance and resistance than the other coil.

20. In a flip-flop lightning ar-rester having at least two parallel circuits each comprising one current limiting gap assembly, means responsive to simultaneous discharge of current through at least two of said parallel circuits for changing the impedance of one of said circuits more rapidly than the impedance of the other parallel circuits,

References Cited UNITED STATES PATENTS 9/1956 Beck 317-61 J D MILLER, Primary Examiner ULYSSES WELDON, Assistant Examiner US. Cl. X.R.

said means comprising electrical impedance devices that 15 31769;74

333 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,538, 387 Dated November 3, 1970 Inventor) Stanley A. Miske, Jr. 3 Eugene C. Sakshaug; James S. K

It is certified that error appears in the above-identified pst ent and that said Letters Patent are hereby corrected as shown below:

Column 3, line '51, 'no" shouldbe not line 70, "-necticl should be -netic "circuit" (second occurrence) should Column line 29,

be current line 50, after the word "resistance" insert the word coil 536Mb A Y SEALED F139 197! ElmilLFktchmJr. mm B. sasunm, JR. flolllssioner of Patents Attutmg Offioer 

